acejos....
Welcome to Aquarium Advice!
The little 'bugs' on your gravel are likely tiny crustaceans (either Cladocerans or some other Copepod). They are not harmful to your fish but a large population of them generally indicates a build-up of uneaten food or other waste in your gravel. If you don't like having so many of them then a regimen of gravel vacuuming should knock the population back to a nearly unnoticeable level.
-Joe